In refitting my Shimano SL-6400-FCAI ("light action" seven speed) downtube shifters the right side one works perfectly with and without SIS selected. However on fitting the left side the lever will not pull towards the seat but will move against spring load towards the front wheel! I just cannot figure out how to "unlock" this lever. The parts can only be assembled one way. Even with the lever off the bike and holding the flatted core piece with pliers the lever will only move anti-clockwise. It was working perfectly before I removed it. It is not possible that the right and left side parts were mistakenly reversed as the mechanisms are different and anyhow the wires are different lengths and have never been removed.

Help!

Ian